1. Field of the Invention
This invention relates generally to a process to assemble disposable personal hygiene products and to the products so assembled, and more particularly to infant and adult disposable diapers and other incontinent devices, such as sanitary napkins and pads, designed to absorb and retain urine or other bodily fluids excreted by the wearer of the product.
2. Status and Description of the Prior Art
The manufacture of disposable hygienic products such as diapers involves the assembly of several components into the final product. Typically, a diaper is assembled from roll goods made from thermoplastic polymers which surround an absorbent layer or component. A standard construction uses a liquid impervious back-sheet made of polyethylene film and a liquid permeable nonwoven cover stock made from polypropylene spunbond or thermally bonded carded fibers. An absorbent pad or system, usually made of fiberized wood pulp combined with superabsorbent materials, is sandwiched between the back-sheet and the coverstock. Other components such as elastomeric threads or bands are also added at the crotch/leg area or the waist band for improving fit and reducing leakage. A secondary leg barrier or cuff may be added to improve the retention of liquid and semi-liquid wastes. The standard method for assembling this system is through the use of hot melt adhesives which are applied by sprays or other hot-melt application methods.
The primary negative aspect of the use of hot melts is the cost of the raw materials. In many applications with complex assembly requirements the cost of hot melt adhesive can become a significant percentage of the cost of the finished product.
An ideal process to join diaper components is to use heat sealing. The process of heat sealing includes both thermal and ultrasonic welding means. It is well known from the physical chemistry of high polymers and from the literature of ultrasonic welding machines that the heat sealing, thermal bonding, joining, or welding of like polymers is a viable practice. It is possible to thermally bond two polyethylene films, a polyethylene film and a polyethylene nonwoven or two polyethylene nonwovens. The same is true of two polypropylene films, a polypropylene film and a polypropylene nonwoven or two polypropylene nonwovens. This viability of heat sealing exists between all homologous polymers. Further the process is not limited to the joining of webs or films but may also be utilized to join elastic members such as are used in the leg areas or waist area of diapers. In fact the process of the instant invention may be used to join any components composed of dissimilar polymers that are present in the product to be assembled.
The process of thermal bonding is well known and is used in many applications in disposables production. A familiar example is the joining or seaming of polyethylene bags. Thermal bonding or thermal welding is accomplished by applying a heated wheel or bar to the materials to be joined. Ultra sonic welding is also available to join similar plastics.
The shortcoming of all of the thermal bonding or welding methods is that these processes are only applicable to the bonding of identical or homologous materials. It is easy to join a polyethylene film to another polyethylene film as in bag manufacturing.
Problems are encountered when dissimilar polymers are used. This problem is related to the lack of solubility between two different high polymers when in the melt state. The dissimilar polymers will not mix and will try to maintain separate domains. This is evident in polymer alloys such as PVC/ABS where the product of mixing two polymers is an alloy rather than a solution. In this case the boundaries between the polymers are very distinct.
Technical literature from Branson (Characteristics And Compatibility Of Thermoplastics For Ultrasonic Assembly) .COPYRGT. Branson Ultrasonics Corporation, 1971 Rev. 4/93 printed USA 6/94) shows that, from their broad industrial experience, technical background and judgment, dissimilar polymers cannot be joined by ultra-sonic (thermal) means.
The instant patent is differentiated from the teachings of U.S. Pat. No. 5,507,895 issued to Suekane which teaches that, for a diaper, two dissimilar polymers may be joined by ultrasonic means. This invention teaches that products utilizing components with dissimilar plastic points may be welded together when the web or film with a higher plastic point is placed next to the heating means to prevent sticking of the web or film to the heating means which may be ultrasonic or thermal. It also limits the invention to the use of an intermittent weld line.